Free-radical Initiated Processes

Free-radical Initiated Processes.—A great deal of work has appeared concerning emulsion copolymerizations such as that of Khanna and Noonan who have described the syntheses of copolymers of acrylic esters with uniform composition by continuous polymerization processes. Emulsion processes will not be considered further here except to mention that the kinetics and current problems in the theory of emulsion polymerization have been reviewed as have many other aspects of this field.  

There is a strong and continuing interest in homogeneous random copolymerizations and radical polymerizations in general have been reviewed.  

Ttldds and Vertes extend their smes of publications on the kinetics of copolymerization with an examination of the dependence of the initiation rate constant on the monomer feed ratio in the copolymerization of acrylonitrile and methyl methacrylate at various temperatures. In measuring the overall rate of polymerization of styrene with ethylacrylate in benzene, Fehervari et al  

Fehervari, T. Foldes-Berezsnich, and F. Tiid6s, Magyar Kem. Lapja, 1977, 32 (11), 561. 

Several authors have examined the pressure effect upon the reactivity of radicals in copolymerization and it is claimed that the direction as well as the magnitude of the pressure effect on the reactivity ratios for some copolymerizations can be predicted by making use of the Q e scheme. Other workers have examined the influence of the solvent in radical copolymerizations. In particular the solvation of radicals and the use of solvation for controlling 

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Features of the free-radical initiation processes are similar for both the homopolymerization of functionalized monomers and copolymerization of the latter with conventional monomers. Common chemical initiators were applied. Azo-bis(isobutyro nitrile) was mostly used in bulk polymerization. No interference with phenolic hydroxy groups was observed in polymerization of 2-hydroxybenzo-phenoiKs, acetophenones, salicylates and of their derivatives [47]. The most rigorous eliinination of oxygen from the reaction mixture was necessary to achieve polymerization of monomeric hindered phenolic antioxidants or derivatives of 2-(2-hydroxyphenyl)benzotriazole [48]. An oxygen-free atmosphere is also an advantage for aromatic amines. A higher initiator level and/or increased temperature appear to be necessary to achieve normal polymerization rates with (D-functionalized monomers [46]. 

For a free radical-initiated process with a monomer, such as ethylene, the three stages can be represented as follows (Eqs. 22.1-22.6). Species suitable for initiation of a free radical-promoted process are normally generated by the homolytic dissociation of weak bonds of a precursor by heat or light (Eq. 22.1). 

If we limit this review to free-radical-initiated processes, step 4 can be a recombination or a disproportionation of growing chains. 

Among the UV sensitizers, some of which may accelerate the crosslinking rate by 100-200 times that of ordinary free-radical initiated processes, are ethers of benzoin, benzophenones, anthrone, benzil, and Michler s ketone [81]. 

Industrial scale manufacture of these polymers involves a free radical initiation process whereby the free radicals are generated by either thermal decomposition or by a redox process. 

These temperature properties determine the polymerization initiation temperature and the length of time the free-radical initiation process is active. For example, low-temperature peroxides possess a relatively short free-radical half-life and offer a low-temperature initiation but also a lower peak temperature, while a high-temperature peroxide with a relatively long... 

This is commonly the case for polymers prepared by free radical initiated processes. 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

Most commercial processes involve copolymerization of ethylene with the acid comonomer followed by partial neutralization, using appropriate metal compounds. The copolymerization step is best carried out in a weU-stirred autoclave with continuous feeds of all ingredients and the free-radical initiator, under substantially constant environment conditions (22—24). Owing to the relatively high reactivity of the acid comonomer, it is desirable to provide rapid end-over-end mixing, and the comonomer content of the feed is much lower than that of the copolymer product. Temperatures of 150—280°C and pressures well in excess of 100 MPa (1000 atm) are maintained. Modifications on the basic process described above have been described (25,26). When specific properties such as increased stiffness are required, nonrandom copolymers may be preferred. An additional comonomer, however, may be introduced to decrease crystallinity (10,27). 

Free-radical polymerization processes are used to produce virtually all commercial methacrylic polymers. Usually free-radical initiators (qv) such as azo compounds or peroxides are used to initiate the polymerizations. Photochemical and radiation-initiated polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of methacrylic monomers is first-order with respect to monomer concentration, and one-half order with respect to the initiator concentration. Rate data for polymerization of several common methacrylic monomers initiated with 2,2 -azobisisobutyronitrile [78-67-1] (AIBN) have been deterrnined and are shown in Table 8. 

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

Etherification and esterification of hydroxyl groups produce derivatives, some of which are produced commercially. Derivatives may also be obtained by graft polymerization wherein free radicals, initiated on the starch backbone by ceric ion or irradiation, react with monomers such as vinyl or acrylyl derivatives. A number of such copolymers have been prepared and evaluated in extmsion processing (49). A starch—acrylonitrile graft copolymer has been patented (50) which rapidly absorbs many hundred times its weight in water and has potential appHcations in disposable diapers and medical suppHes. 

This synthesis method can be utilised by any alkene or alkyne, but steric hindrance on internal double bonds can cause these reactions to be quite slow. Conjugated dienes and aromatic alkenes are not suited for the ultraviolet light-initiated process. The use of other free-radical initiators is required in free-radical-initiated reactions involving these species. 

Initiation. Free-radical initiators are produced by several processes. The high temperatures and shearing stresses required for compounding, extmsion, and molding of polymeric materials can produce alkyl radicals by homolytic chain cleavage. Oxidatively sensitive substrates can react directly with oxygen, particularly at elevated temperatures, to yield radicals. 

Degenerate Explosion it was a free radical autocatalytic process and control was difficult, but manageable. The main disadvantage was that it produced as much or more acrolein as propylene oxide. Because no market existed for acrolein at that time, the project was abandoned. Within two years, the acrylic market developed and a new project was initiated to make acrolein and acrylic acid by vapor-phase catalytic oxidation of propylene. 

Because most widely used methods used to prepare classical styrene/divinylben-zene copolymers have always been based on suspension polymerization, it seemed logical that a series of porous PDVB gels using similar methodologies could be developed. In suspension polymerization, divinylbenzene is suspended as a dispersion of small droplets in a continuous phase of water and polymerized by classical free radical initiation. This process produces the spherical beads... 

Note, Added in Proof-. In their study of the autoxidation of 2-butyl-isoindoline, Kochi and Singleton showed that 2-butylisoindole is formed and is converted by further oxidation to 2-butylphthalimide and 2-butylphthalimidine. The rate of oxidation of 2-butylisoindoline to the isoindole was found to be markedly dependent on hydrogen donor ability of the solvent and was shoivn to involve a free radical chain process. Autoxidation of 2-butylisoindole also appears to be a radical process since it can initiate autoxidation of 2-butylisoindoline. 

Water-soluble free radical initiators (i.e., potassium persulfate, K2S2O8) are used in the emulsion polymerization process. Upon heating, the persulfate ion decomposes into two sulfate ion free radicals according to the following reaction ... 

The free radical initiators are more suitable for the monomers having electron-withdrawing substituents directed to the ethylene nucleus. The monomers having electron-supplying groups can be polymerized better with the ionic initiators. The water solubility of the monomer is another important consideration. Highly water-soluble (relatively polar) monomers are not suitable for the emulsion polymerization process since most of the monomer polymerizes within the continuous medium, The detailed emulsion polymerization procedures for various monomers, including styrene [59-64], butadiene [61,63,64], vinyl acetate [62,64], vinyl chloride [62,64,65], alkyl acrylates [61-63,65], alkyl methacrylates [62,64], chloroprene [63], and isoprene [61,63] are available in the literature. 

PE produced by a high-pressure polymerization process (pressure 1000-3000 atm) using a free radical initiator is a highly branched material that contains both LCBs and SCBs. The polymer so produced is a low-density material (density up to about 0.925 g/cc) and is known as high-pressure low-density PE (HP LDPE). The LCBs are formed via intermolecular hydrogen transfer [19], whereas SCBs are formed by intramolecular hydrogen abstraction [16]. 

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